17 votes
Accepted

Why do we use decibels in radio?

We use decibels because they are the most intuitive representation when our primary concern is signal quality. Signal quality changes not in proportion to the power added (in watts), but rather to the ...
Phil Frost - W8II's user avatar
14 votes

How does IQ modulation work (intuitively)?

I think it's more intuitive if you unlearn some things first. Oscillation is not: $$ \cos(\omega t) $$ where $\omega$ is the angular frequency in radians per second, and $t$ is time. Rather, ...
Phil Frost - W8II's user avatar
12 votes
Accepted

How does signal strength relate to bandwidth?

It depends on what you mean by "stronger". Ignoring actual limitations of your hardware and just considering the theory of communications, if you have 25 W of transmit power, then you can spread that ...
Kevin Reid AG6YO's user avatar
  • 24.5k
11 votes

Is FM effectively spread spectrum?

This is the sort of question that often comes down to “what definition of the word is useful” — not necessary any objective truth. However, I would be inclined to say that no, wider FM should not be ...
Kevin Reid AG6YO's user avatar
  • 24.5k
7 votes
Accepted

How does a Faraday cage block an EMP?

Faraday cages block EMP in the same way they block all other time-varying electromagnetic fields. The only difference between blocking EMP versus blocking an ordinary radio transmission is the EMP is ...
Phil Frost - W8II's user avatar
7 votes
Accepted

Key CW on zero crossings - zero bandwidth?

You raise an excellent question and your thought processes are indeed on the right track. First some background. An ideal, uninterrupted sinusoidal carrier has zero bandwidth. Real world factors such ...
Glenn W9IQ's user avatar
  • 18.6k
6 votes

Can a common-mode current exist on the inside of a coax shield?

It is helpful to understand the basic functioning of a coaxial cable. But first there are two important phenomenons that must be understood in order to proceed. Skin Effect When direct current (time ...
Glenn W9IQ's user avatar
  • 18.6k
6 votes
Accepted

Why do we talk about common mode current from an antenna, but not from a transmitter?

The short answer is that it can't. A shielded transmitter, connected to an ideal piece of coax, does not generate common mode currents. The inner and the outer of the cable look connected (and for ...
tomnexus's user avatar
  • 11.1k
6 votes
Accepted

What does antenna frequency range mean in practice?

It means the range of frequencies in which the antenna is designed to operate. Operated outside that range, the antenna may not meet its specifications. This particular antenna is a telescopic whip, a ...
Phil Frost - W8II's user avatar
6 votes
Accepted

Why do PSK modes have "bandwidth"?

Binary PSK with instantaneous phase shifts would be equivalent to multiplying a sine wave (the carrier) with a square wave with values at 1 or -1. When two signals are multiplied, this forms a ...
Phil Frost - W8II's user avatar
6 votes
Accepted

What is the apparent AC voltage source caused by a mismatched impedance on the antenna or transmission line?

This would usually take a course in Transmission Line Theory, but we can give you the mechanical solution really quickly. Behold: THE SMITH CHART (in impedance form): Wow. A mandala! A fractal! What ...
Marcus Müller's user avatar
5 votes

How does IQ modulation work (intuitively)?

I believe a good point of view is the concept of orthogonality. This is clear under everybody's eyes when seen in physical space, take for instance a 2-dimension space, a plane. In the example above ...
carloc's user avatar
  • 183
5 votes
Accepted

Understanding how antenna mismatch can damage a transmitter

There are a few parts to this, it's good to keep them separate. 1. Antenna impedance Antennas are complicated beasts, they connect the circuit world of voltage, current and power, into the ...
tomnexus's user avatar
  • 11.1k
5 votes
Accepted

Why are there weird sounds in radio?

What you describe is at least in part a feature of single side band demodulation. The radio is taking a range ("window" of bandwidth) of RF spectrum and shifting it to the audio spectrum. ...
user10489's user avatar
  • 6,230
4 votes

Key CW on zero crossings - zero bandwidth?

There is no way to transmit information in a signal with zero bandwidth. Switching the carrier at the zero crossings would reduce bandwidth but not take it to zero. There's a mathematical explanation ...
Phil Frost - W8II's user avatar
4 votes
Accepted

Conventional or electron current

Current flow is electron flow and it is from negative to positive. This is what we call class A hogwash. Current notation is just a convention. Going by electron flow is not righter than going the ...
Marcus Müller's user avatar
4 votes

SDR sampling bandwidth - do the bits per sample matter?

For HF usage with 200 Msps converters and above, an 8 bit ADC will provide very good performance. You write: "the sampling frequency has to be several times the target frequency" but that is not true ...
sm5bsz's user avatar
  • 1,009
4 votes

Can you create a radio beam with a Faraday cage?

I think that if the sphere is very large in wavelengths. Maybe 1000 wl and if the opening is small, maybe 10x10 wl, you would create a fairly narrow beam. Such a small opening in such a big sphere ...
sm5bsz's user avatar
  • 1,009
4 votes
Accepted

Does the E-field propagate faster that the H-field?

The E and H fields of all RF signals travel at exactly the same speed - the speed of light. The phase relationship between the two fields remains constant as they travel through various mediums. ...
Glenn W9IQ's user avatar
  • 18.6k
4 votes
Accepted

Voltage wave in transmission lines

A transmission line is linear, which means we can consider the voltage at any point to be the superposition of two other voltages. Asking what they are "physically" may or may not be a useful ...
Phil Frost - W8II's user avatar
4 votes
Accepted

Transmitting Video Underwater

Here is some useful scholarly information about the signal attenuation (in dB) of EM waves through freshwater at various frequencies and depths. You will find a formula that you can use. You will also ...
Mike Waters's user avatar
  • 7,958
4 votes

How does IQ modulation work (intuitively)?

First let's look at what happens in the case of a single mixer: simulate this circuit – Schematic created using CircuitLab We know that mixers produce sum and difference frequencies. That is to ...
Phil Frost - W8II's user avatar
4 votes

What cause `multiply` has difference result between `float` and `complex` signal?

That's just the math behind it – everything is alright with these results! You need to write down the formula of the real-valued $\sin(t)$ in terms of $e^{j2\pi t}$ and $e^{-j2\pi t}$, and you'll see ...
Marcus Müller's user avatar
4 votes

Is FM effectively spread spectrum?

In a spread spectrum system, you use a method of transporting data in some bandwidth, and then apply some way of spreading it over a larger bandwidth, whilst maintaining the original method of data ...
Marcus Müller's user avatar
3 votes

How can the output power of a Yaesu FT101E be increased?

Should it be confirmed that the old finals are responsible for the low output then there could be only one solution i.e to replace the old finals with new tubes. Thermionic emission from an old ...
vu2nan's user avatar
  • 757
3 votes

How does signal strength relate to bandwidth?

If by "strength" you mean "subjective volume as determined by a human operator", then reducing the bandwidth of the transmission while keeping the transmitter power constant does indeed produce a "...
Phil Frost - W8II's user avatar
3 votes

Beam antenna changed directivity

I would suggest that one of the transmission line connections to the antenna, probably the earth if you are using coax, has become disconnected at the antenna feed point, and it's just a coincidence ...
Andrew's user avatar
  • 3,685
3 votes

Can a common-mode current exist on the inside of a coax shield?

First, does it matter where the common-mode currents flow? Coax works as a transmission line because the (ideally) equal but opposite currents on the center and shield create opposite fields which ...
Phil Frost - W8II's user avatar
3 votes
Accepted

Can you create a radio beam with a Faraday cage?

No, there would not be a narrow beam. There's multiple ways you can show that, but most intuitively, I think is the follow: consider every point on that patch to be a source of an elemental wave in ...
Marcus Müller's user avatar
3 votes
Accepted

Why are tuning and amplification in the same block?

In general, when you see a block diagram, the point of it is that the blocks contain multiple or unspecified elements, but the diagram accurately represents the boundaries of, and connections between, ...
Kevin Reid AG6YO's user avatar
  • 24.5k

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